Performance enhancing additive material for the nickel hydroxide positive electrode in rechargeable alkaline cells

ABSTRACT

A conductive additive for the positive nickel electrode for electrochemical cells which provides increased performance by suppressing an oxygen evolution reaction occurring parallel to the oxidation of nickel hydroxide, increasing conductivity of the electrode and/or consuming oxygen produced as a result of the oxygen evolution reaction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation of, and is entitled to thebenefit of the earlier filing date and priority of, co-pending U.S.patent application Ser. No. 10/428,547, to Ovshinsky et al., which isassigned to the same assignee as the current application, entitled“PERFORMANCE ENHANCING ADDITIVE MATERIAL FOR THE NICKEL HYDROXIDEPOSITIVE ELECTRODE IN RECHARGEABLE ELECTROCHEMICAL CELLS”, filed May 2,2003, the disclosure of which is hereby incorporated by reference. Thepresent invention is a continuation-in-part of, and is entitled to thebenefit of the earlier filing date and priority of, co-pending U.S.patent application Ser. No. 10/378,586, to Ovshinsky et al., which isassigned to the same assignee as the current application, entitled“PERFORMANCE ENHANCING ADDITIVE MATERIAL FOR THE NICKEL HYDROXIDEPOSITIVE ELECTRODE IN RECHARGEABLE ELECTROCHEMICAL CELLS”, filed Mar. 3,2003, the disclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to electrodes utilizingperformance enhancing additives. More particularly, the presentinvention relates to utilizing a performance enhancing additive toincrease power output in a rechargeable electrochemical cell by reducingpositive electrode resistance.

BACKGROUND

The recent trend for portable devices has increased the needs andrequirements for high energy density and high power density rechargeablebatteries. High energy density and high power density are also importantcriteria for batteries used for electric or hybrid vehicles.

Nickel hydroxide has been used for years as an active material for thepositive electrode of alkaline electrochemical cells. Examples of suchnickel-based alkaline cells include nickel cadmium (Ni—Cd) cells,nickel-iron (Ni—Fe) cells, nickel-zinc (Ni—Zn) cells, and nickel-metalhydride (Ni-MH) cells. The energy density of nickel-basedelectrochemical cells may be increased by closely packing the nickelhydroxide active material into an electrically conductive substrate suchas a porous foam. However, there are limitations on the amount ofpressure used to increase packing density. Application of too muchpressure causes expansion of electrode plates and compresses theseparators placed between the positive and negative electrodes. Theovercompression of the cells limit the wettability as well as the amountof electrolyte in separators by squeezing out the absorbed electrolyte,which in turn deteriorates the performance of these cells.

In general, nickel-metal hydride (Ni_MH) cells utilize a negativeelectrode comprising a metal hydride active material that is capable ofthe reversible electrochemical storage of hydrogen. Examples of metalhydride materials are provided in U.S. Pat. Nos. 4,551,400, 4,728,586,and 5,536,591 the disclosures of which are incorporated by referenceherein. The positive electrode of the nickel-metal hydride cellcomprises a nickel hydroxide active material. The negative and positiveelectrodes are spaced apart in the alkaline electrolyte.

Upon application of an electrical current across a Ni-MH cell, the Ni-MHmaterial of the negative electrode is charged by the absorption ofhydrogen formed by electrochemical water discharge reaction and theelectrochemical generation of hydroxyl ions:

The negative electrode reactions are reversible. Upon discharge, thestored hydrogen is released to form a water molecule and release anelectron.

The charging process for a nickel hydroxide positive electrode in analkaline electrochemical cell is governed by the following reaction:

After the first charge of the electrochemical cell, the nickel hydroxideis oxidized to form nickel oxyhydroxide. During discharge of theelectrochemical cell, the nickel oxyhydroxide is reduced to form betanickel hydroxide as shown by the following reaction:

The charging efficiency of the positive electrode and the utilization ofthe positive electrode material is affected by the oxygen evolutionprocess which is controlled by the reaction:2OH—→H₂0+1/2 0₂+2e−  (4)During the charging process, a portion of the current applied to theelectrochemical cell for the purpose of charging, is instead consumed bya parallel oxygen evolution reaction (4). The oxygen evolution reactiongenerally begins when the electrochemical cell is approximately 20-30%charged and increases with the increased charge. The oxygen evolutionreaction is also more prevalent with increased temperatures. The oxygenevolution reaction (4) is not desirable and contributes to lowerutilization rates upon charging, can cause a pressure build-up withinthe electrochemical cell, and can upon further oxidation change thenickel oxyhydroxide into its less conductive forms. One reason bothreactions occur is that their electrochemical potential values are veryclose. Anything that can be done to widen the gap between them (i.e.,lowering the nickel reaction potential in reaction (2) or raising thereaction potential of the oxygen evolution reaction (4)) will contributeto higher utilization rates. It is noted that the reaction potential ofthe oxygen evolution reaction (4) is also referred to as the oxygenevolution potential.

Furthermore, the electrochemical reaction potential of reaction (4) ishighly temperature dependent. At lower temperatures, oxygen evolution islow and the charging efficiency of the nickel positive electrode ishigh. However, at higher temperatures, the electrochemical reactionpotential of reaction (4) decreases and the rate of the oxygen evolutionreaction (4) increases so that the charging efficiency of the nickelhydroxide positive electrode drops.

One way to increase the electrochemical potential of equation (4) is byadding certain additives with the nickel hydroxide active material whenforming the positive electrode material. U.S. Pat. Nos. 5,466,543,5,451,475, 5,571,636, 6,017,655 6,150,054, and 6,287,726 disclosecertain additives which improve the rate of utilization of the nickelhydroxide in a wide temperature range. The present invention disclosesan improved additive which enhances performance of the positiveelectrode by reducing the resistance within the nickel electrode andsimultaneously increasing the oxygen evolution potential.

SUMMARY OF THE INVENTION

The present invention discloses an active material composition for anickel positive electrode comprising a nickel hydroxide material and anadditive material comprising a metal oxide. The metal oxide may be oneor more of a single metal oxide containing a metal from the groupcomprising Ce, Ti, Mo, V, W, Sn, Mn, In, Y, Sm, and Nb, or a binary orhigher non-stoichiometric, solid solution, oxide containing two metalsfrom the group comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, Nb, Ce, andMm.

Preferably, the metal oxide includes cerium and/or titanium. Ceriumoxide, as an additive to the positive electrode, may be further dopedwith a divalent oxide, a trivalent oxide, a tetravalent oxide, orcombinations thereof to improve the overall electrode performance. Thetitanium oxide may also be comprised of two or more sub-oxides havingthe formula TiO_(x), wherein x may range from 0.65 to 1.25.

The single oxide preferably has the formula A_(n)O_(2n-1), wherein4.0≦n≦10 and A is a metal from the group comprising Ti, Mo, V, W, Sn,Mn, In, Y, Sm, or Nb. The binary or higher non-stoichiometric, solidsolution, oxide preferably has the formula B_(x)(A_(n)O_(2n-1))_(1-x),wherein 0.0<x<1.0, 4.0≦n≦10, A is one or more metals from the groupcomprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, or Nb, and B is one or moremetals from the group comprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, Nb,Ce, and Mm.

The active material composition may comprise 78.6 to 85.6 weight percentnickel hydroxide, 3.0 to 6.0 weight percent cobalt, 3.0 to 8.0 weightpercent cobalt oxide, 3.0 to 10.0 weight percent of additive material,and 0.4 weight percent binder material. Preferably, the active materialcomprises 5.0 to 9.0 weight percent of the additive material.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1, shows the conductivities of various titanium oxides ranging frompure metallic Ti to TiO₂.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a positive electrode active materialcomprising a nickel hydroxide material and a performance enhancingadditive material. The performance enhancing additive material inaccordance with the present invention may be comprised of one or moresingle, binary, or ternary or higher oxides. The additives in accordancewith the present invention enhance performance in nickel hydroxidepositive electrodes by suppressing oxygen evolution by increasing theoxygen evolution potential. The additive material may also furtherimprove performance of the positive electrode by increasing conductivitywithin the positive electrode and/or consuming at least some of theevolved oxygen within the electrochemical cell.

In the embodiments of the present invention, the positive electrodeactive material generally comprises 73.6 to 88.1 weight percent nickelhydroxide, 3.0 to 6.0 weight percent cobalt, 3.0 to 8.0 weight percentcobalt oxide, 0.5 to 15.0 weight percent of additive material, and 0.4weight percent binder material. Preferably, the additive material ispresent in the range of 5.0 to 9.0 weight percent.

The nickel hydroxide material may be any nickel hydroxide material knownin the art. It is within the spirit and intent of this invention thatany and all kinds of nickel hydroxide materials may be used. Examples ofpossible nickel hydroxide materials are provided in U.S. Pat. Nos.5,348,822, 5,637,423, and 6,086,843 the disclosure of which are hereinincorporated by reference.

The binder materials may be any material, which binds the activematerial together to prevent degradation of the electrode during itslifetime. Binder materials should be resistant to the conditions presentwithin the electrochemical cells. Examples of binder materials, whichmay be added to the active composition, include, but are not limited to,polymeric binders such as polyvinyl alcohol (PVA), fluoropolymers,carboxymethyl cellulose (CMC), hydroxycarboxymethyl cellulose (HCMC),and mixtures thereof. An example of a fluoropolymer ispolytetrafluoroethylene (PTFE). Other examples of additional bindermaterials, which may be added to the active composition, includeelastomeric polymers such as styrene-butadiene rubber latex.Furthermore, depending upon the application, additional hydrophobicmaterials may be added to the active composition.

In a first embodiment of the present invention, the additive materialmay be cerium oxide. Cerium oxide is essentially nonconductive material,however, cerium oxide increases the oxygen evolution potential, therebysuppressing the parallel oxygen evolution reaction, and consumes atleast some of the oxygen formed by the parallel oxygen evolutionreaction, thus preventing further oxidation of the nickel positiveelectrode materials into nonconductive oxides.

In a second embodiment of the present invention, the additive materialmay comprise a single oxide formed from a metal selected from the groupcomprising Ti, Mo, V, W, Sn, Mn, In, Y, Sm, or Nb. These oxides areelectroconductive and are able to reduce to some extent the overallresistance within the nickel positive electrode by increasing theconductivity and suppressing the parallel oxygen evolution reactionwithin the nickel positive electrode. Oxides of V, Ti, Mo, or W may formoxides in the Magneli Phase having the general formula A_(n)O_(m)wherein n is in the range of 4 to 50 and A is a metal chosen from thegroup of Ti, Mo, V, and W. When A is Ti or V, m equals 2n-1, and when Ais Mo or W, m equals 3n-1 or 3n-2. Metallic oxides in the Magneli phasemay exhibit exceptional electroconductive properties as compared toother metallic oxides. Depending on the metal valency, these metals mayform oxides comprising one or more sub-oxides having differentconductivities. For example, Ti forms an oxide comprised of multiplesub-oxides having different conductivities. Titanium sub-oxides havingthe formula TiO_(x), with 0.65≦x≦1.25 have high conductivities and arepreferred for use in the present invention. A plot showing therelationship between conductivity and stoichiometry of several differenttitanium oxides is shown in FIG. 1.

In a third embodiment of the present invention, the additive materialmay be a mixed oxide which is a non-stoichiometric, solid solution,binary or higher oxide. The non-stoichiometric, solid solution, binaryoxides are formed from oxides of two or more metals selected from thegroup consisting of Ce, Mn, Co, Ni, Sm, Y, Nb, In, Ti, V, W, Mo, and Mm,wherein Mm is a Misch metal alloy formed from two or more of the rareearth metals. As a general rule, these oxides are represented by aformula MO₂ where M represents the sum total of all metals present inthe oxide including the dopants. If it is a true binary, MO₂ willrepresent the total of two metals present in the alloy. Nonstoichiometry in the oxides can be due to oxygen vacancy or metalvacancy in the oxide. In typical examples of TiO₂, in one case there isoxygen vacancy and in the other case there is oxygen excess. Both causedisturbances in the d-orbitals causing disorder in their lattices. Thisdisorder (or non stoichiometry) results in enhanced conductivity andother benefits. When the binary or higher oxides are formed with V, Ti,Mo, or W, the oxides may fall within the Magneli phase. The formula forbinary or higher oxides in the Magneli phase is B_(x)(A_(n)O_(m))_(1-x),wherein 0.0<x<1.0, 4.0≦n≦10, A is one or more metals selected from thegroup of Ti, Mo, V, W, and B is one or more metals chosen from the groupof Ti, Mo, V, W, Sn, Mn, In, Y, Sm, Nb, Ce, and Mm. When A is either Vor Ti, m is equal to 2n-1 and when A is either Mo or W, m is equal to3n-1 or 3n-2. Some elements such as Nb, when included in binary oxideincrease the thermal and electrochemical stability to the binary oxidethereby preventing further oxidation of the binary oxide to lesserconductive forms. Preferably the binary oxides include cerium therebyproviding increased conductivity, increased oxygen evolution potential,and consumption of at least some of the evolved oxygen within the nickelpositive electrode. Niobium and cerium also increase the oxygenvacancies within the oxides. Examples of some of the preferrednon-stoichiometric, solid solution, binary compounds including cerium orniobium are Ce_(0.4)Ti_(3.6)O₇, Ce₂Ti₄O₁₀, Ce₂Ti₅O₁₂, CeTi₄O₉, CeTi₅O₁₁,and Nb_(0.1)Ti_(0.9)O₂.

In a fourth embodiment of the present invention, the additive materialmay be a compositional mixture of two or more oxides selected fromcerium oxide, single oxides, and binary or higher oxides as describedabove.

The additive material suppresses the oxygen evolution reaction byincreasing the electrochemical over potential of the oxygen evolutionreaction thereby making the nickel hydroxide oxidation the morefavorable reaction within the electrode. Without addition of theadditive material the overlapping range of the potential of the oxygenevolution reaction and the potential for the nickel hydroxide oxidationoccurring during charging of the positive electrode is very large. Theadditive material raises the potential of the oxygen evolution reactionthereby reducing the overlapping range between the fully chargedpositive electrode potential and the potential for the oxygen evolutionreaction.

As compared to other additives used to suppress the parallel oxygenevolution reaction occurring within the positive electrode, such asCa(OH)₂, the additive materials in accordance with the present inventionhave a much higher conductivity resulting in lower overall resistancewithin the electrode. The excellent conductivity of the additivematerials create increased conductivity within the positive electrodethereby providing higher power output for the resulting electrochemicalcell.

Additive materials including cerium oxide provide an additional benefitby being able to consume at least some of the evolved oxygen via a redoxcouple mechanism. By consuming at least some of the evolved oxygen viaan oxidation reaction, the redox couple prevents further oxidation ofnickel oxyhydroxide and oxides contained in the additive material toless conductive forms. Cerium oxide, acts as a redox couple (Ce₂O₃⇄CeO₂)by consuming oxygen produced by the parallel oxygen evolution reactionoccurring during charging of the electrochemical cell. Oxygen isconsumed by the cerium oxide redox couple via an oxidation reactionduring charging and is released by the redox couple via a reductionreaction to form hydroxide ions during discharging. The redox couplereaction for cerium oxide is shown as:

The mechanism behind the extraordinary ability of ceria (CeO₂) to store,release, and transport oxygen can be explained by oxygen-vacancyformation and migration coupled with the quantum process of electronlocalization. Ceria releases oxygen under reduction conditions forming aseries of reduced oxides with stoichiometric cerium oxide (Ce₂O₃) as anend product, which in its turn easily takes up oxygen under oxidizingconditions, turning the Ce₂O₃ back into ceria. The CeO₂—Ce₂O₃ transitionis entirely reversible.

Cerium is the first element in the periodic table with a partiallyoccupied f orbital. This leads to many features of elemental cerium,such as the g—a iso-structural transition, where at a critical pressurethe volume of the unit cell suddenly collapses preserving the facecentered cubic (fcc) structure. The reason for this drastic change involume at the transition point can be explained by the delocalization(or metallization) of the 4f electron under pressure.

This characteristic of cerium appears to be equally justified for theinsulating cerium oxides, as cerium formally has the valance 4+ in CeO₂,the most oxidized form of cerium, and 3+ in Ce₂O₃, the other extremefinal state of the transition. In CeO₂, all four valence electrons ofCe, 6s²5d¹4f¹, nominally leave the host atoms and transfer into the pbands of two oxygen atoms, while in Ce₂O₃ the Ce f electron is fullylocalized. The oxygen p band has two extra electrons provided by cerium.These electrons are left behind when an oxygen atom leaves its latticeposition. The oxygen-vacancy formation process is essentiallyfacilitated by a simultaneous condensation of these two electrons intolocalized f-level traps on two cerium (3+) atoms. Such a description ofthe two forms of cerium oxide, that the localization-delocalization ofthe Ce 4f electron is involved in the CeO₂—Ce₂O₃ transition, can also besupported from structural point of view on the microscopic level.

It is possible to choose a common unit cell for both cerium oxides. TheC-type structure of Ce₂O₃, which is the end product of reduction processof CeO₂, can be constructed out of eight unit cells of CeO₂, with 25%oxygen vacancies ordered in a particular way. The addition or removal ofoxygen atoms involves a minimal reorganization of the skeletonarrangement of cerium atoms. This structural property definitelyfacilitates the excellent reversibility of the reduction-oxidationprocess. The condensation of the f electron into core state of a Ce atom(i.e. its localization) leads to 10% volume increase. In other words, asfar as the cerium atoms are concerned, the reduction-oxidationtransition can be viewed upon as an almost isostructural transitionaccompanied by a 10% volume change, in resemblance with the g—atransition in pure fcc cerium showing a volume discontinuity of about16%.

Clearly, on the microscopic level, the removal of an oxygen atom is madepossible due to ability of the cerium atom to easily and drasticallyadjust its electronic configuration to best fit its immediateenvironment. Thus, the process of oxygen-vacancy formation is closelycoupled with the quantum effect of localization/delocalization of the 4felectron of cerium. This is the basis for the oxygen storage capacity ofcerium oxide.

To help promote the beneficial effect of the cerium oxide redox couple,the cerium oxide may be doped with divalent or trivalent oxides (or someof the oxides described above) to create additional structural defectscausing more oxygen vacancies within the redox couple containingelectrode. These additives do not change the fundamental character ofthe reactions but will improve their relative rates. In this aspect theadditives could even be characterized as “promoters”. Solid solutions ofcerium oxide with some oxides such as Y or La can be readily formed. Theresulting intentionally designed oxygen vacancies are mobile and formthe dominant point defect involved in transport behavior; oxygendiffusion is very fast whereas the cation diffusion is slow.

When forming the positive electrode, the positive electrode activematerial is prepared and affixed to a current collector grid. Theadditive materials may be chemically impregnated into the activematerial, mechanically mixed with the active material, co-precipitatedinto or onto the surface of the active material from a precursor,distributed throughout the active material via ultrasonic homogenation,deposited onto the active material via decomposition techniques, orcoated onto the active material. The positive electrode active materialmay be formed into a paste, powder, or ribbon. The positive electrodeactive material may also be pressed onto the current collector grid topromote additional stability throughout the electrode. The currentcollector grids in accordance with the present invention may be selectedfrom, but not limited to, an electrically conductive mesh, grid, foam,expanded metal, perforated metal, or combination thereof. The mostpreferable current collector grid is an electrically conductive meshhaving 40 wires per inch horizontally and 20 wires per inch vertically,although other meshes may work equally well. The wires comprising themesh may have a diameter between 0.005 inches and 0.01 inches,preferably between 0.005 inches and 0.008 inches. This design providesoptimal current distribution due to the reduction of the ohmicresistance. Where more than 20 wires per inch are vertically positioned,problems may be encountered when affixing the active material to thesubstrate. One current collector grid may be used in accordance with thepresent invention, however the use of two current collector grids mayfurther increase the mechanical integrity of the positive nickelelectrode.

EXAMPLE 1

Several test cells using positive electrodes in accordance with thepresent invention were constructed and tested against a cell using astandard (control) positive electrode. To form the standard positiveelectrode, a standard positive electrode paste was formed from 88.6weight percent nickel hydroxide material with co-precipitated zinc andcobalt from Tanaka Chemical Company, 5.0 weight percent cobalt, 6.0weight percent cobalt oxide, and 0.4 weight percent polyvinyl alcoholbinder. The paste was then affixed to a current collector grid to formthe standard positive electrode. Three additional positive electrodeswere constructed similarly by replacing 3.0 weight percent of the nickelhydroxide material with cerium oxide, 5.0 weight percent nickelhydroxide with cerium oxide, and 10.0 weight percent nickel hydroxidewith cerium oxide respectively.

A positive limited tri-electrode battery cell (test battery cell) wasformed using two hydrogen storage alloy negative electrodes, a nickelhydroxide positive electrode and an auxiliary Hg/HgO referenceelectrode. Each one of these electrodes are contained in a nonconducting but porous separator bag to prevent shorting. The hydrogenstorage alloy negative electrode includes an active electrodecomposition formed by physically mixing 97 wt % of a hydrogen storagealloy, 1.0 wt % carbon, and 2.0 wt % binder. The active electrodecomposition is made into a paste and applied onto a current collectorgrid to form the negative electrode.

After the initial formation procedure and two regular charge/dischargecycles, the control cell (utilizing standard positive nickel electrode)and the test cells (utilizing positive nickel electrode with ceriumoxide) are each discharged to 50% depth of discharge at constantdischarge current. The control cell and the test cells are thensubjected to a sequence of 10 and 30 second discharge pulses ofincreasing magnitude (0.5 amp, 1 amp, 1.5 amp, etc.). The potentialchange (ΔV) of the positive electrode after 10 and 30 seconds ismeasured relative to the Hg/HgO reference electrode. The positiveelectrode potential values in respect to Hg/HgO reference electrode (atthe end of each of the discharge current pulses) were plotted versus thevalue of the discharge currents for both the control cell and the testcells. The slopes of the linear portion of the plots represent theresistance of each positive nickel electrode. The electrodes containingthe cerium oxide showed reduced resistance as compared to the standardpositive electrode tested under the same conditions. The resistancegradually decreased with the increase of cerium oxide up to 10 percentby weight of the positive electrode active material. The results forthese tests are shown in Table 1. TABLE 1 Resistance at the Resistanceat the end of a 10 end of a 30 Sample Second Pulse Second Pulse StandardElectrode .065 Ohm .072 Ohm Electrode w/3 wt % CeO₂ .059 Ohm .065 OhmElectrode w/5 wt % CeO₂ .049 Ohm .054 Ohm Electrode w/10 wt % CeO₂ .047Ohm .052 Ohm

EXAMPLE 2

Several test cells using a positive electrodes in accordance with thepresent invention was constructed and tested against a cell using astandard (control) positive electrode. To form the standard positiveelectrode, a standard positive electrode paste was formed from 88.6weight percent AP64 nickel hydroxide material produced by Ovonic BatteryCompany with co-precipitated zinc and cobalt, 5.0 weight percent cobalt,6.0 weight percent cobalt oxide, and 0.4 weight percent polyvinylalcohol binder. The paste was then affixed to a current collector gridto form the standard positive electrode. Three electrodes in accordancewith the present invention were constructed similarly by replacing someof the nickel hydroxide material with an additive material in accordancewith the present invention. One electrode was constructed with 5.0weight percent of the nickel hydroxide material being replaced withcommercially available EBONEX (the registered trademark of AtraverdaLimited) titanium oxide. A second electrode was constructed with 5.0weight percent of the nickel hydroxide material being replaced withsynthesized Magneli phase type titanium oxide with the overall formulaTi₄O₇. A third electrode was constructed with 5.0 weight percent of thenickel hydroxide material being replaced with a first sample ofCe_(0.4)Ti_(3.6)O₇ produced using a one step synthesis process. A fourthelectrode was constructed with 5.0 weight percent of the nickelhydroxide material being replaced with a second sample of the sameformulation Ce_(0.4)Ti_(3.6)O₇ produced using a two step synthesisprocess.

A sample of Magneli phase type titanium oxide (Ti₄O₇) was synthesized bycombining an appropriate amount of TiO₂ powder and Ti powder and heatingthe well mixed homogeneous mixture for 4 hours at 1300° C. in a hydrogenenvironment. A first sample of Ce_(0.4)Ti_(3.6)O₇ was produced utilizinga one step process including combining appropriate amounts of CeO₂,TiO₂, and Ti, mixing the materials into a homogeneous mixture, andheating the mixture for 4 hours at 1200° C. in a hydrogen environment. Asecond sample of Ce_(0.4)Ti_(3.6)O₇ was produced utilizing a two stepsynthesis process including 1) combining TiO₂ and Ti and heating for 4hours at 1300° C. in a hydrogen environment to form Ti₄O₇, and 2) addingCeO₂ and heating for an additional 4 hours at 1300° C. in a hydrogenenvironment to form Ce_(0.4)Ti_(3.6)O₇.

As described in example 1, a positive limited tri-electrode battery cell(test battery cell) was prepared using two hydrogen storage alloynegative electrodes, a nickel hydroxide positive electrode and anauxiliary Hg/HgO reference electrode. Each one of these electrodes arecontained in a non conducting but porous separator bag to preventshorting. The hydrogen storage alloy negative electrode includes anactive electrode composition formed by physically mixing 97 wt % of ahydrogen storage alloy, 1.0 wt % carbon, and 2.0 wt % binder. The activeelectrode composition is made into a paste and applied onto a currentcollector grid to form the negative electrode.

After the initial formation procedure and two regular charge/dischargecycles, the control cell (utilizing standard positive nickel electrode)and the test cell (utilizing positive nickel electrode with additivematerial in accordance with the present invention) are each dischargedto 50% depth of discharge at constant discharge current. The controlcell and the test cells were then subjected to a sequence of 30 seconddischarge pulses of increasing magnitude (0.5 amp, 1 amp, 1.5 amp, etc.)The potential change (ΔV) of the positive electrode after 10 and 30seconds is measured relative to the Hg/HgO reference electrode. Thepositive electrode potential values in respect to Hg/HgO referenceelectrode (at the end of each of the discharge current pulses) wereplotted versus the value of the applied discharge currents for both thecontrol cell and the test cell. The slopes of the linear portion of theplots represents the resistance of each positive nickel electrode. Theelectrodes containing the additives in accordance with the presentinvention showed reduced resistance as compared to the standard positiveelectrode tested under the same conditions. The electrodes containingthe Ce_(0.4)Ti_(3.6)O₇ formed by the two step process exhibited the bestresults. The results for these tests are shown in Table 2. TABLE 2Resistance at the Resistance at the end of a 10 end of a 30 SampleSecond Pulse Second Pulse Standard Electrode .065 Ohm .071 Ohm Electrodew/5 wt % Ebonex .056 Ohm .062 Ohm Ti₄O₇ Electrode w/5 wt % .046 Ohm .053Ohm synthesized Ti₄O₇ additive Electrode w/5 wt % .051 Ohm .059 OhmCe_(0.4)Ti_(3.6)O₇ (1 step process) Electrode w/5 wt % .041 Ohm .047 OhmCe_(0.4)Ti_(3.6)O₇ (2 step process)

It is to be understood that the disclosure set forth herein is presentedin the form of detailed embodiments described for the purpose of makinga full and complete disclosure of the present invention, and that suchdetails are not to be interpreted as limiting the true scope of thisinvention as set forth and defined by the appended claims.

1. An active material composition for a nickel positive electrodecomprising: a nickel hydroxide material; and an additive materialcomprising a solid solution, said solid solution comprising two or moremetals.
 2. The active material composition according to claim 1, whereinsaid solid solution additive lacks nickel.
 3. The active materialcomposition according to claim 1, wherein said solid solution additiveincludes two or more metals having an oxidation state of +2 or higher.4. The active material composition according to claim 1, wherein saidsolid solution additive includes two or more metals having an oxidationstate of +3 or higher.
 5. The active material composition according toclaim 1, wherein said solid solution additive is a metal oxide, saidmetal oxide incorporating said two or more metals.
 6. The activematerial composition according to claim 1, wherein said solid solutionadditive does not form a solid solution with said nickel hydroxidematerial.
 7. The active material composition according to claim 1,wherein the structure of said solid solution additive differs from thestructure of said nickel hydroxide material.
 8. The active materialcomposition according to claim 1, wherein said solid solution additivecomprises cerium.
 9. The active material composition according to claim1, wherein said solid solution additive comprises three or more metals.10. The active material composition according to claim 1, wherein saidnickel hydroxide material further comprises Co.